1. Field of the Invention
The present invention relates to a flameless stream reformer.
2. Description of the Prior Art
Generally, a reformer refers to a device that produces hydrogen from fossil fuels or hydrocarbon fuels for use in a fuel cell.
The reformer uses fuel reforming technology to produce hydrogen from fuels such as methane, gasoline or methanol and has been frequently used in petrochemical processes such as ammonia synthesis processes. However, it is difficult to apply to direct fuel cells, because of limitations caused by the characteristics of the fuel cells.
Fuel cells for large-scale power generation are operated at a relatively fixed output level and have a short shutdown time, and thus can use natural gas reformers which have been used in existing chemical factories. However, the reformer needs to be miniaturized depending on the output of the fuel cell.
Fuel cells for home use have small capacity and are operated at various output levels.
It is known that solid oxide fuel cells are operated at a high temperature of 600˜1000° C. and a hydrocarbon fuel injected directly into the fuel electrode of the fuel cell can be internally reformed into H2 (fuel for the fuel cell) and CO.
However, for this purpose, a reforming catalyst needs to be provided in the fuel cell stack. Also, when the reforming catalyst is inactivated, the body of the fuel cell should be replaced.
Particularly, in internal reforming, carbon deposition is highly likely to occur throughout the anode (for hydrocarbons having many carbon atoms, the possibility of carbon deposition is high), and in this case, the inactivation of the reforming catalyst can rapidly occur.
When a steam reforming reaction occurs rapidly, a severe drop in temperature can locally occur to deteriorate the performance of the fuel cell and cause severe thermal stress in the fuel cell structure, thus shortening the life expectancy of the fuel cell.
Due to the above-described problems, studies on the use of external reformers in the operation of solid oxide fuel cells (SOFCs) are being actively conducted.
Current methods of reforming hydrocarbon fuels to produce hydrogen can be classified into decomposition, steam reforming, partial oxidation, and autothermal reforming.
The decomposition reaction of hydrocarbons is a reaction in which hydrocarbons are decomposed into hydrogen and carbon by heating in a state in which water or oxygen is not introduced. This decomposition reaction has an advantage in that pure hydrogen can be obtained using a relatively simple process.
However, when a complex hydrocarbon is decomposed, there are shortcomings in that reaction by-products in addition to carbon and hydrogen are produced and carbon is continuously deposited in the reactor to reduce the activity of the catalyst and block the gas passage in the reformer.
For this reason, as reforming methods that are to be applied to a general SOFC fuel cell system, steam reforming, partial oxidation reforming and autothermal reforming have been considered.
In the steam reforming method in which fuel and steam are allowed to react with each other, oxygen for converting carbon in a hydrocarbon into carbon monoxide is supplied from water to the hydrocarbon, while a reaction for obtaining hydrogen from water occurs. The basic reactions for the steam reforming method are expressed in the following reaction schemes (1-1) and (1-2):
(1-1) (1-2)
Thus, it is known that the steam reforming reaction produces hydrogen in an amount larger than the amount of hydrogen contained in a hydrocarbon, and thus has high hydrogen production efficiency.
However, this steam reforming reaction has a problem in that it is an intense endothermic reaction in which a portion of fuel should be burned to supply heat for the reaction such that the high hydrogen production efficiency of the reaction can be partially offset.
In addition, it is known that, because the reactor for steam reforming should additionally be provided with a burner or the like, it is large and complex, and thus is frequently applied to processes for large-scale hydrogen production. Also, the reactor is unsuitable for use as a small-scale reformer due to its long starting time.
Meanwhile, the partial oxidation reforming reaction is a reaction in which carbon in a hydrocarbon is converted into carbon monoxide using either pure oxygen or oxygen contained in air. It is divided into a catalytic partial oxidation reaction using a catalyst and a non-catalytic partial oxidation reaction.
The non-catalytic partial oxidation reaction progresses at a high temperature of 1150˜1400° C., whereas the catalytic partial oxidation reaction progresses at a relatively low temperature of 700˜850° C., and thus does not need to use a reactor made of an expensive material.
The partial oxidation reforming reaction is a very fast and weakly exothermic reaction in which heat for the reaction does not need to be supplied from the outside. Thus, the reactor for the reaction is small in size and has significantly excellent starting characteristics.
Unlike the steam reforming method, the partial oxidation reforming reaction is an exothermic reaction that starts fast. Also, it has high response characteristics even when the amount of hydrogen supplied is changed according to a change in the load of the fuel cell.
However, the partial oxidation reforming reaction requires high temperature, and the purity of hydrogen obtained thereby is low, indicating that it has energy efficiency lower than the steam reforming reaction.
Meanwhile, the autothermal reforming is a method in which the steam reforming reaction that is endothermic and the partial oxidation reaction that is exothermic occur at the same time in the presence of the same catalyst such that the reaction heat becomes zero.
For example, when each of the reactions uses methane as a raw material, the steam reforming reaction is strongly endothermic, and the partial oxidation reaction is weakly exothermic. Thus, in order for the sum of the reactions to become thermally neutral, the ratio of the steam reforming reaction to the partial oxidation reforming reaction should become 1:5.7.
The nature of the autothermal reforming reaction does not significantly differ from that of the partial oxidation reforming reaction. Thus, when the extent of the steam reforming reaction in the autothermal reforming reaction is increased, heat for the reaction should be supplied from the outside, and thus the reactor becomes more complex and larger and the operation of the reaction is difficult to control. For this reason, the autothermal reforming reaction is mainly used in small-scale systems.
Heat transfer to a reforming catalytic layer occurs mainly by the burner flame, and heat transfer from the burner flame is required to preheat fuel or steam. Thus, controlling the burner flame to a suitable shape and size is required and an igniter is also required.
The burner flame is essential in terms of the structure of a reformer, but the burner flame can cause the problem of oxidizing the combustion chamber wall depending on operating conditions.
A reformer according to the prior art will now be described with reference to FIG. 1.
Referring to FIG. 1, below a reformer 10 according to the prior art, a burner 11 is disposed. A flame supplied from the burner 11 to the reformer 10 moves directly toward the inner wall of the combustion chamber of the burner while it can damage the inner wall of the combustion chamber.
For this reason, hot spots can easily occur on the inner wall of the combustion chamber while they can deteriorate the durability of the combustion chamber.
Also, a reforming catalyst 12 provided in the reformer 10 needs to be easily replaced due to its limited life expectancy. However, the reformer 10 according to the prior art has a problem in that it is difficult to replace the reforming catalyst 12.
Another reformer according to the prior art will now be described with reference to FIG. 2.
Referring to FIG. 2, a burner 11 is disposed below a reformer 20. A flame generated from the burner 11 heats the lower outer portion of the reformer 20 so as to heat a reforming catalyst 12.
The flame from the burner is concentrated on a specific portion of the reformer 20 while it deteriorates the durability of the reformer 20. Thus, there is a need to solve this problem.